The higher the stones’ content of this mineral, the less likely it is that surgery will clear them

Patients with renal stones containing calcium phosphate (CP) are less likely to be rendered stone-free following percutaneous nephrolithotomy (PNL) than patients who have non-CP-containing stones, data suggest.

Robert B. Nadler, MD, and his colleagues at Northwestern University’s Feinberg School of Medicine in Chicago analyzed 213 PCNL procedures performed on 111 patients. Patients with non-CP stones were five times as likely to be rendered stone-free following PNL compared with patients harboring CP-containing stones, the investigators reported in The Journal of Urology (2008;180:958-960). Patients were considered stone-free if fragments were 2 mm in maximum diameter or less on non-contrast CT.

In addition, PNL success rates decreased with increasing CP content, independent of preoperative stone burden, the authors said. Patients with stones containing less than 60% and 80% CP were, respectively, 2.4 and 3.7 times more likely to be rendered stone-free as patients having stones with a greater than 60% and 80% CP content.

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The researchers said that the current study, to their knowledge, provides the first data correlating CP stone composition and PNL-associated stone-free rate.

“Given the results of our study, it appears that difficult stone breakage may be a function of stone composition and not necessarily related to treatment modality,” the investigators wrote. This decreased success rate, they noted, does not appear to be related to patient gender, age, or stone burden.

As for why CP stones may be more difficult to clear with PNL, the investigators suggested that CP stones may be more adherent to the renal papilla or parenchyma, making it difficult to remove all stones completely during the procedure.

In an accompanying editorial, J. Stuart Wolf Jr, MD, of the University of Michigan in Ann Arbor, noted that CP “is an important marker for medical treatment for stones. Stones that are predominately CP indicate an increased likelihood of renal tubular acidosis and primary hyperparathyroidism.”

CP exists in two forms, apatite and brushite, Dr. Wolf noted. The mechanism underlying the inverse relationship between increasing CP content and PNL success, he said, may be related to resistance of brushite to fragmentation, pre-existing incorporation of apatite into renal tissue, or other unknown factors.